1. Field of the Invention
The present invention relates to refractory materials which up to now could not be melted without corroding or melting the receptacle which contained them. This is the case of metals such as tungsten (melting point 3400.degree. C.), molybdenum (melting point 2600.degree. C.), or refractory oxides and synthetic materials (silicides, carbides . . . ).
2. Description of the Prior Art
Up to the present time solid synthetic refractory materials (in the non powdery state) have been obtained either by sintering, or by melting in a consumable crucible.
Sintering generally consists in agglomerating powders, and heating the agglomerated work pieces so as to form "sintering bridges" between the particles and material. In the case of metals, they may then be hammered or rolled when hot until solid work pieces are obtained having the theoretic density of the metal. This is the technique used for obtaining tungsten or molybdenum work pieces.
Another sintering technique is used for producing cermets. This is liquid phase sintering. This technique is used for producing tungsten carbide work pieces. The powdery refractory carbides (CW, TIC, TAC) are mixed with cobalt powder. The mixtures are compressed and heated to the melting temperature of the cobalt. After cooling, the carbides are bonded by a ductile and tough cobalt film.
The production of molten tungsten carbide practiced up to now forms the "hot crucible" type of fusion. It takes place in the following way: a powdery mixture of carbon and tungsten is fed into a graphite crucible. The walls of this crucible are heated by induction by subjecting it to the action of a high frequency magnetic field, or by any other heating method. The crucible is then itself heated to a high temperature and its walls raise the temperature of the mixture of powders by thermal conduction. The powder mixture reaches the melting temperature by conduction. This temperature is close to 2750.degree. C. That requires a good external insulation of the crucible, whose walls must be heated to an even higher temperature.
This method has numerous drawbacks, particularly:
the amount of carbon in the alloy obtained is difficult to control for a part of the graphite of the crucible passes into the alloy; and for this reason the crucible is gradually consumed, PA1 continuous production cannot be contemplated because of the method of heating and because of heating the crucible itself.
Experience shows that melting a material such as tungsten carbide in a hot crucible requires very rapid discharge of the material from the crucible after melting, so as to obtain a satisfactory structure (see U.S. Pat. No. 1,839,518). That is incompatible with a continuous process. In fact, keeping a liquid mass of molten tungsten carbide for a sufficient time in a graphite crucible causes a progressive increase of the carbon content of the tungsten carbide. This increase involves an increase of the melting temperature. Consequently, it is very difficult to keep the contents of a crucible for the time necessary for a low even flow for spraying under good conditions.
Furthermore, induction heating techniques in a "cold crucible" are known, in which techniques a solid work piece is heated directly by subjecting it directly to the action of an alternating magnetic field. By "cold crucible" is meant a crucible in which the walls are designed for transmitting and concentrating the magnetic field without themselves being subjected to appreciable heating by induced currents. Such a type of "cold crucible" is described more particularly in the patent FR-A-2 036 418. However, it has proved difficult to heat and a fortiori to melt refractory powdery mixtures directly in such a "cold crucible". In fact, calculations show that for a grain size of a few tenths of a millimeter, for the tungsten powder for example, a magnetic field frequency higher than 1 MHz must be used. It is difficult to produce sufficient power at this frequency whose use is moreover likely to jam radiowaves. Such a solution does not then seem directly applicable in industry.
The aim of the present invention is more particularly to overcome the drawbacks of the known methods and devices, by providing means for directly heating the refractory powder mixture by magnetic induction.